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Abstract:

A prime mover revolution speed control system for a hydraulic
construction machine sets the revolution speed of the prime mover in
accordance with the operating state invoked by an operating command and
as a result of a determination of an excavation state so that that the
revolution speed of the engine can be increased for a heavy load
(speedup) in the excavation state. When the control lever is fully
operated in the direction of arm crowding, the control lever 43 is also
operated, and first judgment conditions are all met to conclude that an
excavation state has begun. During excavation work, when the control
lever 44 is subjected to a half or greater operation in the arm crowding
direction, second judgment conditions are all met to conclude that the
excavation state persists, and the speedup sequence is continued.

Claims:

1. A prime mover revolution speed control system for a hydraulic
construction machine, the prime mover revolution speed control system
comprising: a prime mover; at least one variable-displacement hydraulic
pump that is driven by the prime mover; a plurality of hydraulic
actuators that are driven by hydraulic fluid of the hydraulic pump; a
plurality of control valves that control the flow rate and flow direction
of the hydraulic fluid delivered from the hydraulic pump to the hydraulic
actuators; operating command means that is provided for each of the
hydraulic actuators to issue operating commands for the hydraulic
actuators in accordance with the amount of operation in each operation
direction by driving the control valves on an individual basis; operating
command detection means that detects the operating commands of the
operating command means; pump pressure detection means that detects the
pressure of the hydraulic pump; standard target revolution speed setup
means that sets a standard target revolution speed for the prime mover;
target revolution speed correction means that acquires a target
revolution speed for the prime mover by adding a predetermined correction
value to the standard target revolution speed in accordance with an
operating state invoked by the operating command means; and excavation
state identification means that judges, in accordance with the operation
direction, the operation amount, and the pump pressure, whether an
excavation state prevails; wherein, when the excavation state
identification means concludes that an excavation state prevails, the
target revolution speed correction means adds the predetermined
correction value to the standard target revolution speed.

2. The prime mover revolution speed control system according to claim 1,
wherein, if, in a situation where the result of the last judgment does
not indicate that an excavation state prevails, first judgment conditions
are met as the pump pressure is a predetermined first threshold value or
higher, an arm crowding operation amount is a value equivalent to a full
operation or greater, and at least either a boom raising operation amount
or a bucket crowding amount is a minimum operation amount or greater, the
excavation state identification means concludes that the excavation state
has begun; wherein, if , in a situation where the result of the last
judgment does not indicate that the excavation state prevails, the first
judgment conditions are not met, the excavation state identification
means concludes that a non-excavation state has persisted; wherein, if,
in a situation where the result of the last judgment indicates that the
excavation state prevails, second judgment conditions are met as the pump
pressure is a second threshold value or higher, which is lower than the
first threshold value, and the arm crowding operation amount is a value
equivalent to a half operation or greater, the excavation state
identification means concludes that the excavation state has persisted;
and wherein, if , in a situation where the result of the last judgment
indicates that the excavation state prevails, the second judgment
conditions are not met, the excavation state identification means
concludes that the excavation state has terminated.

Description:

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a prime mover control system for a
hydraulic construction machine. The invention more particularly relates
to a prime mover revolution speed control system for a hydraulic
excavator or other hydraulic construction machine that includes an engine
as a prime mover and performs necessary work by driving a hydraulic
actuator with hydraulic fluid discharged from a hydraulic pump
rotationally driven by the engine.

[0003] 2. Description of the Related Art

[0004] Hydraulic construction machines such as a hydraulic excavator
generally include an engine as a prime mover. The engine rotationally
drives at least one variable-displacement hydraulic pump. Hydraulic fluid
discharged from the hydraulic pump drives a plurality of hydraulic
actuators to perform excavation work or other necessary tasks. The
hydraulic excavator includes a setup means for specifying a target
revolution speed of the engine. A fuel injection amount is then
controlled in accordance with the target revolution speed to provide
revolution speed control.

[0005] The above-described revolution speed control system generally
exercises control to provide a constant revolution speed. From the
viewpoint of work efficiency enhancement and fuel consumption rate
improvement, however, it may be preferable in some cases that the
revolution speed of the prime mover be increased in accordance with load.
In particular, when excavation work, which is likely to impose a heavy
load, is performed, it is preferred that the revolution speed of the
engine be higher than when a light load is imposed (speedup).

[0006] A revolution speed control system described, for instance, in
Japanese Patent No. 2905324 includes operating state judgment means and
prime mover revolution speed correction means. The operating state
judgment means judges, in accordance with an operation of a control lever
(existence of operation or absence of operation), the direction of
control lever operation, and the pressure of the hydraulic pump, whether
the current operating state needs an increase in the prime mover
revolution speed. The prime mover revolution speed correction means
increases the prime mover revolution speed by a predetermined value in
accordance with the operating state determined by the operating state
judgment means.

[0007] The operating state judgment means includes revolution speed
correction maps A-C that are selected in accordance with detection
results indicative of an operation of a control lever for each actuator
(existence or absence) and the direction of control lever operation. When
excavation work is performed, an arm crowding operation and a bucket
crowding operation are detected. Then, the map C associated with the
result of detection is selected. When the map C is selected, the engine
revolution speed is first increased by 100 rpm without regard to the pump
pressure. When the pump pressure exceeds 20 MPa (first threshold value),
the engine revolution speed is gradually increased by up to 500 rpm. When
the pump pressure drops below 17 MPa (second threshold value), the
increase in the engine revolution speed is gradually decreased to 100
rpm.

SUMMARY OF THE INVENTION

[0008] However, the related art described above has the following
problems.

[0009] When the hydraulic pump is used for excavation work, its pressure
may greatly vary with an excavation target and an operator's operating
procedure. If the hydraulic pump pressure varies outside the range
between the first threshold value and the second threshold value during
the use of the related art, a speedup sequence repeatedly begins and ends
so that the operator feels uncomfortable with a machine operation. This
results in decreased operational performance. The decrease in the
operational performance disturbs the operator's concentration, thereby
reducing the effect of work efficiency enhancement based on speedup.

[0010] When the related art is used, excavation work and non-excavation
work are differentiated by setting the first and second threshold values.
The first threshold value corresponds to the beginning of excavation
work, whereas the second threshold value corresponds to the end of the
excavation work. If the first and second threshold values are decreased
without a grounded reason, unexpected speedup may occur during other work
but excavation. As a precautionary measure, therefore, the first and
second threshold values are set to be relatively high for safety
assurance.

[0011] However, if the first and second threshold values are set to be
unduly high, a speedup sequence begins in the middle of excavation work
(speedup control does not readily start) and ends during excavation work
(speedup control readily becomes ineffective). Consequently, the effect
of work efficiency enhancement based on speedup cannot be fully obtained.

[0012] The present invention has been made in view of the above
circumstances and provides a prime mover revolution speed control system
that is used for a hydraulic construction machine and capable of
identifying an excavation state with increased certainty to improve the
operational performance during excavation work and enhance the work
efficiency.

[0013] (1) In order to solve the above problems, according to one aspect
of the present invention, there is provided a prime mover revolution
speed control system for a hydraulic construction machine, the prime
mover revolution speed control system including a prime mover, at least
one variable-displacement hydraulic pump, a plurality of hydraulic
actuators, a plurality of control valves, operating command means,
operating command detection means, pump pressure detection means,
standard target revolution speed setup means, target revolution speed
correction means, and excavation state identification means. The
hydraulic pump is driven by the prime mover. The hydraulic actuators are
driven by hydraulic fluid of the hydraulic pump. The control valves
control the flow rate and flow direction of the hydraulic fluid that is
delivered from the hydraulic pump to the hydraulic actuators. The
operating command means is provided for each of the hydraulic actuators
to issue operating commands for the hydraulic actuators in accordance
with the amount of operation in each operation direction by driving the
control valves on an individual basis. The operating command detection
means detects the operating commands of the operating command means. The
pump pressure detection means detects the pressure of the hydraulic pump.
The standard target revolution speed setup means sets a standard target
revolution speed for the prime mover. The target revolution speed
correction means acquires a target revolution speed for the prime mover
by adding a predetermined correction value to the standard target
revolution speed in accordance with an operating state invoked by the
operating command means. The excavation state identification means
judges, in accordance with the operation direction, the operation amount,
and the pump pressure, whether an excavation state prevails. When the
excavation state identification means concludes that an excavation state
prevails, the target revolution speed correction means adds the
predetermined correction value to the standard target revolution speed.

[0014] The related art causes the excavation state identification means to
judge, in accordance with the operation direction, an operation performed
or not (existence or absence), and the pump pressure, whether an
excavation state prevails. On the other hand, the present invention
causes the excavation state identification means to judge, in accordance
with the operation direction, the operation amount, and the pump
pressure, whether an excavation state prevails. This makes it possible to
judge with increased certainty whether an excavation state prevails.

[0015] When the related art is used, relatively high threshold values
(safety side values) are set for the pump pressure because the related
art may not always be able to judge with certainty whether an excavation
state prevails. On the other hand, the present invention identifies an
excavation state with increased certainty. Therefore, the present
invention makes it possible not only to assure safety, but also to set
the threshold values for the pump pressure to be lower than when the
related art is used.

[0016] When the related art is used, the threshold values for the pump
pressure are set to be relatively high. Therefore, when the pump pressure
varies outside the range between the threshold values, speedup and
non-speedup sequences repeatedly occur to the detriment of operational
performance. The present invention, on the other hand, sets low threshold
values for the pump pressure. This provides speedup at all times and
improves the operational performance.

[0017] When the related art is used, relatively high threshold values are
set for the pump pressure. Therefore, the speedup sequence begins during
excavation work and ends during excavation work. In other words, the
range of speedup is narrow so that the effect of work efficiency
enhancement based on speedup cannot be fully obtained. The present
invention, on the other hand, sets low threshold values for the pump
pressure. This ensures that the speedup sequence starts at the beginning
of excavation work and terminates at the end of excavation work.
Consequently, the range of speedup is wider than the related art so that
improved work efficiency results.

[0018] (2) According to another aspect of the present invention, there is
provided the prime mover revolution speed control system as described in
(1) above, wherein, if, in a situation where the result of the last
judgment does not indicate that an excavation state prevails, first
judgment conditions are met as the pump pressure is a predetermined first
threshold value or higher, an arm crowding operation amount is a value
equivalent to a full operation or greater, and at least either a boom
raising operation amount or a bucket crowding amount is a minimum
operation amount or greater, the excavation state identification means
concludes that the excavation state has begun; wherein, if the first
judgment conditions are not met in a situation where the result of the
last judgment does not indicate that the excavation state prevails, the
excavation state identification means concludes that a non-excavation
state has persisted; wherein, if, in a situation where the result of the
last judgment indicates that the excavation state prevails, second
judgment conditions are met as the pump pressure is a second threshold
value or higher, which is lower that the first threshold value, and the
arm crowding operation amount is a value equivalent to a half operation
or greater, the excavation state identification means concludes that the
excavation state has persisted; and wherein, if the second judgment
conditions are not met in a situation where the result of the last
judgment indicates that the excavation state prevails, the excavation
state identification means concludes that the excavation state has
terminated.

[0019] Consequently, the excavation state identification means included in
the present invention is capable of judging with increased certainty
whether the excavation state prevails.

[0020] As the present invention identifies an excavation state with
increased certainty, it makes it possible to improve the operational
performance during excavation work and enhance the work efficiency.

[0025]FIG. 5A is a diagram illustrating the relationship between pump
pressure changes PD and speedup that prevails during the use of the
related art.

[0026]FIG. 5B is a diagram illustrating the relationship between pump
pressure changes PD and speedup that prevails during the use of an
embodiment of the present invention (first advantage).

[0027]FIG. 6A is a diagram illustrating the relationship between pump
pressure changes PD and speedup that prevails during the use of the
related art.

[0028]FIG. 6B is a diagram illustrating the relationship between pump
pressure changes PD and speedup that prevails during the use of the
embodiment of the present invention (second advantage).

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Constitution

[0029] An embodiment of the present invention will now be described with
the accompanying drawings. In the embodiment described below, it is
assumed that the present invention is applied to a hydraulic excavator.

[0030]FIG. 1 is an external view of the hydraulic excavator. The
hydraulic excavator includes a lower travel structure 100, an upper swing
structure 101, and a front work device 102. The lower travel structure
100 includes right- and left-hand travel motors 15, 16. The travel motors
15, 16 rotationally drive a crawler 106 so that the hydraulic excavator
travels forward or backward. The upper swing structure 101 includes a
swing motor 11. The swing motor 11 causes the upper swing structure 101
to swing clockwise or counterclockwise relative to the lower travel
structure 100. The front work device 102 includes a boom 103, an arm 104,
and a bucket 105. The boom 103 is moved up and down by a boom cylinder
13. The arm 104 is operated toward a dumping position (opening position)
or a crowding position (scooping position) by an arm cylinder 12. The
bucket 105 is operated toward the dumping or crowding position by a
bucket cylinder 14.

[0031]FIG. 2 is a schematic diagram illustrating a hydraulic system for
the hydraulic excavator. The hydraulic system includes an engine 1,
variable-displacement hydraulic pumps 2, 3, a pilot pump 4, a plurality
of hydraulic actuators 11-16, a plurality of control valves 21-28, and
operating command devices 41-44. The variable-displacement hydraulic
pumps 2, 3 are driven by the engine 1. The hydraulic actuators 11-16 are
driven by hydraulic fluid of the hydraulic pumps 2, 3. The control valves
21-28 control the flow rate and flow direction of the hydraulic fluid
that is supplied from the hydraulic pumps 2, 3 to the hydraulic actuators
11-16. The operating command devices 41-44 are provided for the hydraulic
actuators 11-16 to issue operating commands to the hydraulic actuators
11-16 in accordance with the amount of operation in each operation
direction by driving the control valves 21-28 with the hydraulic fluid of
the pilot pump 4.

[0033] The control valves 21-24 are positioned on a center bypass line
that is connected to the hydraulic pump 2. The control valve 21 is for
the swing motor 11. The control valve 22 is for the arm cylinder 12. The
control valve 23 is for the boom cylinder 13. The control valve 24 is for
the left-hand travel motor 15. The control valves 25-28 are positioned on
a center bypass line that is connected to the hydraulic pump 3. The
control valve 25 is for the arm cylinder 12. The control valve 26 is for
the boom cylinder 13. The control valve 27 is for the bucket cylinder 14.
The control valve 28 is for the right-hand travel motor 16.

[0034] The arm cylinder 12 is provided with two control valves 22, 25. The
boom cylinder 13 is also provided with two control valves 23, 26.
Hydraulic fluid flows from the two hydraulic pumps 2, 3 can converge for
supply purposes. The operating command device 41 is a left-hand travel
control pedal. Operating pilot pressures TR1, TR2 are generated in
accordance with the operation direction and operation amount of the
control pedal 41 to provide switching control of the control valve 24.
The operating command device 42 is a right-hand travel control pedal.
Operating pilot pressures TR3, TR4 are generated in accordance with the
operation direction and operation amount of the control pedal 42 to
provide switching control of the control valve 28. The operating command
device 43 is a control lever for both the boom and bucket. Operating
pilot pressures BOD, BOU are generated in accordance with one operation
direction and the operation amount of the control lever 43 to provide
switching control of the control valves 23, 26. Operating pilot pressures
BKD, BKC are generated in accordance with the other operation direction
and the operation amount of the control lever 43 to provide switching
control of the control valve 27. The operating command device 44 is a
control lever for both the arm and swing. Operating pilot pressures ARD,
ARC are generated in accordance with one operation direction and the
operation amount of the control lever 44 to provide switching control of
the control valves 22, 25. Operating pilot pressures SW1, SW2 are
generated in accordance with the other operation direction and the
operation amount of the control lever 44 to provide switching control of
the control valve 21.

[0036]FIG. 3 is a conceptual diagram illustrating a control system that
controls the engine and the hydraulic system. The control system includes
a vehicle body controller 51 and an engine controller 52.

[0037] The vehicle body controller 51 provides electronic control of the
hydraulic system for the hydraulic excavator. For example, the vehicle
body controller 51 detects the operation amounts of the operating command
devices 41-44 through the pressure sensors 47-49, and provides tilt
control of the hydraulic pumps 2, 3 to ensure that their discharge flow
rates correspond to the detected operation amounts. Further, the vehicle
body controller 51 computes a target revolution speed NR of the engine 1
in accordance, for instance, with a command signal from an engine control
dial 53, and outputs the computed target revolution speed NR to the
engine controller 52.

[0038] The engine controller 52 inputs an actual revolution speed NE from
a revolution speed sensor 54, which is included in the engine 1 to detect
the actual revolution speed NE of the engine 1, and outputs a fuel
injection command to an electronic governor 55 in the engine 1 so that
the actual revolution speed NE remains equal to the target revolution
speed NR.

[0039] An excavation state identification function 62 and a target
revolution speed correction function 64, which are peculiar to a prime
mover revolution speed control system according to the present
embodiment, are parts of processing functions of the vehicle body
controller 51.

[0042] The standard target revolution speed setup function 61 computes a
standard target revolution speed NR0 of the engine 1 in accordance with a
command signal from the engine control dial 53. The relationship between
the command signal from the engine control dial 53 and the standard
target revolution speed NR0 of the engine 1 is predefined so that the
standard target revolution speed NR0 increases with an increase in the
command signal (voltage) from the engine control dial 53.

[0043] In accordance with the operation directions and operation amounts
of the operating command devices 41-44 and the pump pressures of the
hydraulic pumps 2, 3, the excavation state identification function 62
judges whether an excavation state prevails. If the result of judgment
indicates that an excavation state prevails, the excavation state
identification function 62 outputs a coefficient of 1.0. If, on the other
hand, the judgment result does not indicate that an excavation state
prevails, the excavation state identification function 62 outputs a
coefficient of 0.0. Further, the excavation state identification function
62 outputs the judgment result to itself. Details of excavation state
identification will be described later.

[0044] The correction value setup function 63 presets a correction value
ΔN. For the sake of explanation, the present embodiment assumes
that the correction value ΔN remains unchanged. However, the
correction value ΔN may vary with the pump pressures of the
hydraulic pumps 2, 3.

[0045] The target revolution speed correction function 64 multiplies the
correction value ΔN, which is preset by the correction value setup
function 63, by the coefficient output from the excavation state
identification function 62, adds the resulting value to the standard
target revolution speed NR0, which is computed by the standard target
revolution speed setup function 61, to determine the target revolution
speed NR, and outputs the determined target revolution speed NR to the
engine controller 52.

[0046] The details of excavation state identification by the excavation
state identification function 62 will now be described.

[0047] The excavation state identification function 62 inputs the result
of the last judgment from itself. Further, the excavation state
identification function 62 inputs the discharge pressure PD1 of the
hydraulic pump 2 from the pressure sensor 45, inputs the discharge
pressure PD2 of the hydraulic pump 3 from the pressure sensor 46, and
regards the average of the discharge pressure PD1 and discharge pressure
PD2 as a pump pressure PD. The pump pressure PD is filtered by a filter
65 to cancel the influence of a pump surge pressure generated at startup.
This makes it possible to avoid an unnecessary increase in the engine
revolution speed. The excavation state identification function 62 inputs
the operating pilot pressure ARC, which corresponds to the arm crowding
operation amount, from the pressure sensor 47, the operating pilot
pressure BOU, which corresponds to the boom raising operation amount,
from the pressure sensor 48, and the operating pilot pressure BKC, which
corresponds to the bucket crowding operation amount, from the pressure
sensor 49.

[0048] If, in a situation where the result of the last judgment does not
indicate that an excavation state prevails, first judgment conditions are
met as the pump pressure PD is a predetermined first threshold value (13
MPa) or higher, the arm crowding operation amount ARC is full (2.5 MPa)
or greater, and at least either the boom raising operation amount BOU or
the bucket crowding operation amount BKC is a value corresponding to an
operated state (the operated state corresponds to a minimum operation
amount, for example, 0.7 MPa, at which a boom raising operation or a
bucket crowding operation is started by operating a control lever) or
greater, it is concluded that an excavation state prevails (an excavation
state has begun).

[0049] If, in a situation where the result of the last judgment does not
indicate that an excavation state prevails, at least one of the first
judgment conditions is not met, that is, if the pump pressure PD is lower
than the predetermined first threshold value (13 MPa), the arm crowding
operation amount ARC is lower than a value (2.5 MPa) equivalent to a full
operation, or at least either the boom raising operation amount BOU or
the bucket crowding operation amount BKC is lower than the minimum
operation amount (0.7 MPa), it is concluded that an excavation state does
not prevail (a non-excavation state has persisted).

[0050] If, in a situation where the result of the last judgment indicates
that an excavation state prevails, second judgment conditions are met as
the pump pressure PD is a second threshold value (10 MPa) or higher and
the arm crowding operation amount ARC is a value (1.5 MPa) equivalent to
a half operation or higher, it is concluded that an excavation state
prevails (an excavation state has persisted).

[0051] If, in a situation where the result of the last judgment indicates
that an excavation state prevails, at least one of the second judgment
conditions is not met, that is, if the pump pressure PD is lower than the
second threshold value (10 MPa) and the arm crowding operation amount ARC
is lower than a value (1.5 MPa) equivalent to a half operation, it is
concluded that an excavation state does not prevail (an excavation state
has terminated).

Correspondence with Claims

[0052] The control pedals 41, 42 and control levers 43, 44, which are
provided respectively for the hydraulic actuators 11-16, constitute
operating command means, which issues operating commands for the
hydraulic actuators 11-16 in accordance with the amount of operation in
each operation direction by driving the control valves 21-28 on an
individual basis. The pressure sensors 47, 48 constitute operating
command detection means, which detects the operating commands of the
control levers 43, 44. The pressure sensors 45, 46 constitute pump
pressure detection means, which detects the pressures of the hydraulic
pumps 2, 3.

[0054] An excavation operation performed by the prime mover revolution
speed control system according to the present embodiment will now be
described.

[0055] Before the start of excavation work, the control levers 43, 44 are
not operated, the pump pressure PD is minimized, and none of the first
judgment conditions are met. Thus, the excavation state identification
function 62 concludes that an excavation state does not prevail, and
outputs a coefficient of 0.0. The target revolution speed correction
function 64 sets the standard target revolution speed NR0 as the target
revolution speed NR. Obviously, the prime mover revolution speed control
system does not provide speedup.

[0056] When an operator intends to initiate excavation work and operates
the control lever 44 in the direction of arm crowding, the arm cylinder
12 extends to turn the arm 104 in the arm crowding direction (see FIG.
1). In this instance, the control lever 44 is fully operated so that the
operating pilot pressure ARC, which corresponds to the arm crowding
operation amount, is 2.5 MPa or higher. The procedure for starting the
excavation work involves a combined operation of arm crowding and boom
raising or a combined operation of arm crowding and bucket crowding (see
FIG. 1). Therefore, the control lever 43 is also operated so that either
the boom raising operation amount BOU or the bucket crowding operation
amount BKC is 0.7 MPa or higher.

[0057] Meanwhile, when the arm 104 merely turns in the air, no heavy load
is generated so that the pump pressure PD remains below 13 MPa.
Therefore, the first judgment conditions are not met. Consequently, the
excavation state identification function 62 concludes that a
non-excavation state has persisted, and outputs a coefficient of 0.0. The
target revolution speed correction function 64 sets the standard target
revolution speed NR0 as the target revolution speed NR. The prime mover
revolution speed control system does not provide speedup.

[0058] When the arm 104 turns to let the bucket 105 catch an excavation
target, load is generated to increase the pump pressure PD. When the pump
pressure PD increases to 13 MPa or higher, all the first judgment
conditions are met. The excavation state identification function 62 then
concludes that an excavation state has begun, and outputs a coefficient
of 1.0. The target revolution speed correction function 64 determines the
target revolution speed NR by adding the correction value ΔN to the
standard target revolution speed NR0. In other words, the prime mover
revolution speed control system initiates a speedup sequence.

[0059] When the excavation work is continuously performed, the load
persists so that the pump pressure PD seldom drops below 10 MPa. In most
cases, the control lever 44 is subjected to a half or greater operation
so that the operating pilot pressure ARC corresponding to the arm
crowding operation amount is 1.5 MPa or higher. Therefore, all the second
judgment conditions are met. Consequently, the excavation state
identification function 62 concludes that an excavation state has
persisted, and outputs a coefficient of 1.0. The target revolution speed
correction function 64 determines the target revolution speed NR by
adding the correction value ΔN to the standard target revolution
speed NR0. In other words, the prime mover revolution speed control
system continues with the speedup sequence.

[0060] The operation performed during continued excavation work is not
always a combined operation of arm crowding and boom raising or a
combined operation of arm crowding and bucket crowding. Therefore, the
boom raising operation amount BOU and bucket crowding operation amount
BKC are not the factors of the second judgment conditions.

[0061] When the excavation target has almost been excavated and the load
is reduced so that the pump pressure PD drops below 10 MPa, the second
judgment conditions are not met. Therefore, the excavation state
identification function 62 concludes that the excavation state has
terminated, and outputs a coefficient of 0.0. The target revolution speed
correction function 64 sets the standard target revolution speed NR0 as
the target revolution speed NR. The prime mover revolution speed control
system terminates the speedup sequence.

[0062] If the operator intends to halt the excavation work during
excavation and operates the control lever 44 by less than the half
amount, the operating pilot pressure ARC corresponding to the arm
crowding operation amount drops below 1.5 MPa. As a result, the second
judgment conditions are not met. Therefore, the excavation state
identification function 62 concludes that the excavation state has
terminated, and outputs a coefficient of 0.0. The target revolution speed
correction function 64 sets the standard target revolution speed NR0 as
the target revolution speed NR. The prime mover revolution speed control
system terminates the speedup sequence.

[0063] As described above, the prime mover revolution speed control system
initiates the speedup sequence at the beginning of excavation and
terminates the speedup sequence at the end of excavation.

[0064] Other operations but excavation performed by the prime mover
revolution speed control system according to the present embodiment will
now be described.

[0065] As is the case with excavation work, aerial work (suspending) and
leveling work may also be a combined operation of arm crowding and boom
raising or a combined operation of arm crowding and bucket crowding. When
the control lever 44 is fully operated, the operating pilot pressure ARC
corresponding to the arm crowding operation amount is 2.5 MPa or higher.
Further, when the control lever 43 is operated as well, the boom raising
operation amount BOU or the bucket crowding operation amount BKC is 0.7
MPa or higher.

[0066] Meanwhile, the aerial work (suspending) and leveling work do not
incur heavy load so that the pump pressure PD remains lower than 13 MPa.
Therefore, the first judgment conditions are not met. Consequently, the
excavation state identification function 62 concludes that the
non-excavation state has persisted, and outputs a coefficient of 0.0. The
target revolution speed correction function 64 sets the standard target
revolution speed NR0 as the target revolution speed NR. The prime mover
revolution speed control system does not provide speedup.

Advantage

[0067] Advantages of the present embodiment will now be described in
comparison with those of the related art.

[0068] The revolution speed control system according to the related art
includes the operating state judgment means and the prime mover
revolution speed correction means. The operating state judgment means
judges, in accordance with the operation of the control lever (existence
of operation or absence of operation), the direction of control lever
operation, and the pressure of the hydraulic pump, whether the current
operating state needs an increase in the prime mover revolution speed.
The prime mover revolution speed correction means increases the prime
mover revolution speed by the predetermined value in accordance with the
operating state determined by the operating state judgment means. The
operating state judgment means includes the revolution speed correction
maps that are selected in accordance with detection results indicative of
the operation of the control lever for each actuator (existence or
absence) and the direction of control lever operation. When excavation
work is performed, an arm crowding operation and a bucket crowding
operation are detected. Then, the map corresponding to the result of
detection is selected. In accordance with the selected map, the prime
mover revolution speed control system initiates a speedup sequence when
the pump pressure rises to the first threshold value (20 MPa) or higher,
and terminates the speedup sequence when the pump pressure drops below
the second threshold value (17 MPa). The first and second threshold
values are defined so as to distinguish between excavation work and
non-excavation work. If the first and second threshold values are
decreased without a grounded reason, unexpected speedup may occur during
non-excavation work. As a precautionary measure, therefore, the first and
second threshold values are set to be relatively high for safety
assurance.

[0069]FIG. 5A is a diagram illustrating the relationship between speedup
and changes in the pump pressure PD that prevails during the use of the
related art. The horizontal axis indicates elapsed time and the vertical
axis indicates the pump pressure PD. During excavation work, the pump
pressure PD may greatly vary with the excavation target and the procedure
performed by the operator. If the pump pressure varies outside the range
between the first threshold value and the second threshold value, the
speedup sequence repeatedly begins and ends. The speedup sequence
followed when the pump pressure PD varies as indicated in FIG. 5A will be
described below. When the pump pressure PD is lower than 20 MPa, the
non-speedup sequence is followed. The speedup sequence begins when the
pump pressure PD is 20 MPa or higher, and terminates when the pump
pressure PD is lower than 17 MPa. After the non-speedup sequence is
followed for a while, the speedup sequence begins again and then
terminates.

[0070] When the above-described speedup sequence is followed, the operator
feels uncomfortable with a machine operation. This results in decreased
operational performance. The decrease in the operational performance
disturbs the operator's concentration, thereby reducing the effect of
work efficiency enhancement based on speedup.

[0071]FIG. 5B is a diagram illustrating the relationship between speedup
and changes in the pump pressure PD that prevails during the use of the
present embodiment. The excavation state identification function 62
according to the present embodiment judges, in accordance with the
directions of operations of the control levers 43, 44, their operation
amounts (ARC, BOU, and BKC), and the pump pressures PD of the hydraulic
pumps 2, 3, whether an excavation state prevails. In contrast to the
related art, which merely detects whether the control levers are operated
or not, the present embodiment detects the operation amounts of the
control levers as well. At the beginning of excavation work, the arm
crowding operation amount ARC is equivalent to a full operation and
either a combined operation of arm crowding and boom raising or a
combined operation of arm crowding and bucket crowding is performed.
During continued excavation work, the arm crowding operation amount ARC
is a value equivalent to a half operation or greater. Detecting the
operation amounts makes it possible to judge with increased certainty
whether the excavation state prevails.

[0072] The present embodiment makes it possible to distinguish between
excavation work and non-excavation work with higher certainty than the
related art. Therefore, even when the first and second threshold values
are set to be lower than when the related art is used, the present
embodiment provides the same degree of safety as the related art due to
trade-off. In other words, the first and second threshold values can be
set to be lower than those used with the related art while safety is
assured. For example, the first threshold value is set at 13 MPa in the
present embodiment although it is set at 20 MPa in the related art, and
the second threshold value is set at 10 MPa in the present embodiment
although it is set at 17 MPa in the related art.

[0073] A first advantage of the present embodiment will now be described.

[0074] Changes in the pump pressure PD indicated in FIG. 5B are the same
as changes in the pump pressure PD indicated in FIG. 5A. The non-speedup
sequence is followed while the pump pressure PD is lower than 13 MPa.
When the pump pressure PD is 13 MPa or higher, the speedup sequence
begins and then continues for a while. When the pump pressure PD drops
below 10 MPa, the speedup sequence terminates. As described above, the
prime mover revolution speed control system according to the present
embodiment provides improved operational performance because it
constantly follows the speedup sequence during excavation work. Providing
improved operational performance increases the operator's concentration,
thereby enhancing the effect of work efficiency improvement based on
speedup.

[0075] A second advantage of the present embodiment will now be described.

[0076]FIG. 6A is a diagram illustrating the relationship between speedup
and changes in the pump pressure PD that prevails during the use of the
related art. The pump pressure PD indicated in FIG. 6A is stabler than
the pump pressure PD indicated in FIG. 5A. In this instance, operational
performance problems are not likely to become evident.

[0077] However, the first and second threshold values are set to be
relatively high. Therefore, the speedup sequence begins in the middle of
excavation (speedup control does not readily start) and terminates during
excavation (speedup control readily becomes ineffective). Consequently,
the effect of work efficiency enhancement based on speedup cannot be
fully obtained.

[0078]FIG. 6B is a diagram illustrating the relationship between speedup
and changes in the pump pressure PD that prevails during the use of the
present embodiment. The changes in the pump pressure PD indicated in FIG.
6B are the same as the changes in the pump pressure PD indicated in FIG.
6A.

[0079] In the present embodiment, the first and second threshold values
can be set to be lower than in the related art. Therefore, the present
embodiment initiates the speedup sequence at the beginning of excavation
(speedup control readily starts) and terminates the speedup sequence at
the end of excavation (speedup control does not readily become
ineffective). It means that the speedup sequence persists in the present
embodiment for a longer period than in the related art.

[0080] The concept of speedup sequence period is substituted by the
concept of speedup range and appended to FIG. 1. A dotted arc represents
the range of speedup provided by the related art, and a solid arc
represents the range of speedup sequence provided by the present
embodiment. When the related art is used, the speedup sequence begins in
the middle of excavation and terminates during excavation. Consequently,
the range of speedup is narrow so that the effect of work efficiency
enhancement based on speedup cannot be fully obtained. When, on the other
hand, the present embodiment is used, the speedup sequence starts at the
beginning of excavation and terminates at the end of excavation.
Consequently, the range of speedup is wider than the related art so that
the effect of work efficiency enhancement based on speedup can be fully
obtained. In other words, the present embodiment makes it possible to
provide enhanced work efficiency.